Epoxy compositions are used widely in industry as adhesives, paints, and coatings for a number of applications. Epoxy adhesives are particularly useful for bonding metal surfaces together to provide strong structural bonds. Structural epoxy adhesives have replaced spot welding and other methods of mechanical fastening in many industrial applications, such as in automotive assembly plants.
In some applications, it is often desirable to induction cure an epoxy adhesive composition for a short period of time so that the adhesive develops a green strength sufficient to bond metal surfaces. The induction cure generally involves a short exposure to radio frequency energy, on the order of magnitude of less than a minute, and often less than about 10 seconds, which generates heat in the metal parts to start the curing reaction. During this time, the epoxy adhesive catalyst needs to be activated to start the curing of the adhesive. Curing of the adhesive to its final state, i.e., a structural adhesive, is usually done in a subsequent oven bake.
Catalysts which have sufficient reactivity for induction curing often have a very limited pot life; other catalysts may not effect sufficient curing to build the desired green strength during the induction heating cycle. It is desirable for the epoxy adhesive to build a strong, robust bond to the metal surface, usually through an oily coating of a mill oil, cutting fluid, draw oil, or a combination thereof. A bond is referred to as robust if the bond breaks apart cohesively at high shear values when tested in an overlap shear test. The bonds may break in three different modes: (1) the adhesive splits apart, leaving portions of the adhesive adhered to both metal surfaces in a cohesive failure mode; (2) the adhesive pulls away from either metal surfaces in an adhesive failure mode, or (3) a combination of adhesive and cohesive failure. Optimally, the adhesive fails in either the cohesive failure mode, or a combination of adhesive and cohesive failure.
There remains an ongoing need for improved epoxy adhesives that build robust, cohesive failure bonds to oily metals.